An electrostatic ion trap, herein simply termed an electrostatic trap (EST), employs an electrostatic field to trap ions. Examples of ESTs include the Kingdon trap, Knight trap and the commercial ORBITRAP™ mass analyzer. Other examples of ESTs include numerous types of reflectron electrostatic ion trap including those of planar geometry, or ESTs having a ‘racetrack’ configuration wherein ions are deflected around a circuit multiple times. ESTs are increasingly being employed in mass spectrometry as high-resolution, accurate-mass (HRAM) analysers, as evidenced by the dramatic rise of instrumentation based on the ORBITRAP mass analyzer. Oscillations of the ions trapped in the EST are detected and the oscillation frequencies and/or mass-to-charge ratios (m/z) of the ions are determined, for example by Fourier transformation.
A particular challenge associated with ESTs is the effective injection of ions into the EST. The ORBITRAP mass analyzer utilizes an RF-only straight or curved linear trap (the latter being termed a C-trap) as an ion storage device from which ions are injected into the EST, as described in U.S. Pat. No. 6,872,938. The linear trap is operated to provide pulsed injection of ions into the EST and is implemented using circuitry as described in U.S. Pat. No. 7,498,571. Axial or radial ejection of ions from the linear trap are possible, with radial ejection tending to provide better spatial focusing of ions into the EST in practice. In U.S. Pat. No. 7,425,699 is described an embodiment of ion injection having a so-called liner downstream of a pulsed ion trap that is used for energy lifting but it does not function as an ion guide as it does not have any field inside. Furthermore, the liner does not produce time-of-flight focusing or bunching of ions.
In U.S. Pat. No. 8,796,619 is described an ion injection system for an orbital trapping EST in which ions are released from an ion storage device via a pulsed ion extraction lens. However, there is no separation in time between release of ions from the ion storage device and applying the extraction voltage pulse and the system does not produce a temporal compression of ions upon injection.
Other methods of ion injection have been proposed for use with an EST, such as injection from an orthogonal accelerator (U.S. Pat. No. 6,888,130), injection from a 3D ion trap (U.S. Pat. No. 8,901,491), injection from a gas-filled linear trap with a subsequent orthogonal acceleration from an RF ion guide (WO 2011/086430) and injection via a Kingdon ion guide located in a drill-hole in the wall of a Kingdon ion trap (U.S. Pat. No. 8,907,271).
Another proposed approach is to provide continuous ion injection into the EST with subsequent excitation as described in WO 2008/063497 and WO 2012/092457.
Further approaches envisaged could include mass-dependent ejection from an ion trap as described in WO 2007/027764 and in U.S. Pat. No. 7,582,864, in which an unbalanced linear trap with axial ejection could be combined with an orthogonal accelerator.
It is noted that all methods employing direct ejection from a gas-filled trap to an EST tend to suffer from fragmentation of large molecular ions (e.g. proteins) during ion extraction from the trap. Furthermore, there is a need to provide effective differential pumping in a small space to prevent gas carryover to the EST.
It is therefore desirable to avoid these disadvantages when injecting ions into an EST.